1. Field of the Invention
The present invention relates to a process for preparing a 1,2-pyrazolidine-4-yl-disulfide compound represented by the following formula: ##STR3## wherein R.sup.1 and R.sup.2 are, independently each other, a hydrogen atom or an amino-protecting group,
which is a useful intermediate for producing 6,7-dihydro-6-mercapto-5H-pyrazolo[1,2-a][1,2,4]triazolium.
2. Description of the Prior Art
Heretofore, there have been proposed carbapenem antibiotics which possess potent antimicrobial activity with a broad spectrum, and they have been used in clinical practice. For example, Japanese Laid-Open Patent Publication No. 230,286/1992 discloses (1R,5S,6S)-2-[(6,7-dihydro-5H-pyrazolo[1,2-a][1,2,4]triazolium-6-yl)]thio- 6-[(R)-1-hydroxyethyl]-1-methyl-carbapenum-3-carboxylate as one of such carbapenem compounds.
This carbapenem compound may be prepared by using a mercapto reagent, 6,7-dihydro-6-mercapto-5H-pyrazolo[1,2-a][1,2,4-triazolium derivative of the following formula: ##STR4## wherein X.sup.- is an anion charge. In the above patent publication, this mercapto reagent is prepared by the following Reaction Scheme [A]. ##STR5## In the Reaction Scheme [A], the only process for preparing the starting disulfide compound of the formula (IIa) is disclosed in Japanese Laid-Open Patent Publication No. 230,267/1992. The following Reaction Scheme [B] shows the process. ##STR6## wherein R.sup.3 is bromine atom. However, the process comprises as many as four reaction steps from the starting compound, that is, 1) thioacetylation, 2) hydrolysis, 3) oxidation, and 4) deprotection. Furthermore, the second reaction step gives rise to a thiol compound, which causes irritant bad odor. In view of these disadvantages, there has been a demand for another process for preparing the compound of formula (IIa) which would be preferable and suitable for the purpose of commercial-scale production.